Characterization of the alloreactive helper T-cell response to the platelet membrane glycoprotein IIIa (integrin-beta3) in human platelet antigen-1a alloimmunized human platelet antigen-1b1b women

The prevalence of HPA-1a alloimmunization and the potential risk of FNAIT depend on both the DRB3*01:01 allele and associated DR-DQ haplotypes

Alloimmunization against human platelet antigen (HPA)-1a during pregnancy can cause foetal/neonatal alloimmune thrombocytopenia (FNAIT) and severe bleeding in the foetus or newborn and likely depends on several factors. HPA-1a alloimmunization is associated with DRB3*01:01, which is associated with several DR-DQ haplotypes. However, it is not known to what extent these haplotypes contribute to the prevalence of HPA-1a alloimmunization. HPA-1a-alloimmunized women, identified in a prospective study, and random donors were typed for selected DRB3, DRB4, DRB1, DQA1 and DQB1 alleles to determine allele and DR-DQ haplotype frequencies. DRB3*01:01 was carried by 94% HPA-1a-immunized women compared to 27% in the general population. In the first population, the DR3-DQ2 haplotype was overrepresented (P < .003). The prevalence of HPA-1a alloimmunization was estimated to be about twice as frequent with DR3-DQ2 compared to DR13-DQ6, together accounting for about 90% of DRB3*01:01-positive individuals. Further, we examined DQB1*02 and DRB4*01:01 alleles for their reported association with HPA-1a alloimmunization, in the context of DR-DQ haplotypes. Since ~ 80% of DQB1*02 alleles are linked to the DR3-DQ2 haplotype, the association might be coincidental. However, the DQB1*02:02-associated DR7-DQ2 haplotype was also overrepresented in alloimmunized women, suggesting a role for this allele or haplotype in HPA-1a alloimmunization. As DRB4*01:01 is predominantly associated with the DR7-DQ2 haplotype in HPA-1a-alloimmunized individuals, the reported association with FNAIT may be coincidental. Typing for DR-DQ haplotypes revealed important genetic associations with HPA-1a alloimmunization not evident from typing individual alleles, and the presence of different DRB3-associated DR-DQ haplotypes showed different prevalence of HPA-1a alloimmunization.

Murine models for studying treatment, prevention and pathogenesis of FNAIT

Maternal alloimmunization to paternally inherited antigens on fetal/neonatal platelets can cause fetal/neonatal alloimmune thrombocytopenia (FNAIT) after antibody-mediated removal of platelets from the fetal circulation. The complications vary from mild bleeding symptoms to severe intracranial hemorrhage and subsequent neurological impairment or death. Studies on in vivo mechanisms are challenging to measure directly in pregnant women, rendering murine models as valuable and attractive alternatives, despite some critical differences between mice and men affecting the translational value. Here we present and discuss, the different murine models that substantially have increased our knowledge and understanding of FNAIT pathogenesis - as well as pre-clinical evaluation of therapeutic and preventive strategies.

The prevalence of antibodies against the HLA-DRB3 protein in kidney transplantation and the correlation with HLA expression

Human leukocyte antigen (HLA)-DRB3 is a functional HLA class II gene, which has a limited allele diversity in the human population. Furthermore, the HLA-DRB3 gene is only present in a subset of individuals. Therefore, in organ transplantation, this HLA molecule is frequently mismatched between patient and graft donor and thus antibodies against this mismatched HLA molecule can develop. In this study, we aimed to evaluate the prevalence and reactivity of these antibodies and aimed to identify factors that underlie antibody formation against HLA-DRB3. We showed in our patient cohort that HLA-DRB3 antibodies are identified in about 7% of all patients that were screened with solid phase assays. In these assays, we observed multiple antibody reactivity patterns indicating that HLA-DRB3 harbours multiple epitopes. In those cases, where we succeeded at tracing back the induction of these antibodies to the molecular HLA typing of the immunogenic event, we noticed a different frequency of HLA-DRB1 allele groups in the donors as compared to a control group. To a certain extent this distribution (e.g. HLA-DRB1*11 individuals) could be linked to an altered expression level. However, it also appears that different HLA-DRB3 alleles (e.g. HLA-DRB3*01 group) vary in their immunogenicity without having an expression difference. In conclusion, our study provides information on the immunogenicity and reactivity patterns of antibodies against HLA-DRB3 in kidney transplantation, and it points towards the possibility of HLA expression as a factor underlying antibody formation.

Neonatal alloimmune thrombocytopenia caused by anti-HPA antibodies in pregnant Chinese women: a study protocol for a multicentre, prospective cohort trial

BACKGROUND

Neonatal alloimmune thrombocytopenia (NAIT), caused by maternal antibodies raised against alloantigens carried on foetal platelets, is a very common haematological abnormality in newborns worldwide. However, baseline data on NAIT in China are lacking. Therefore, this study seeks to explore the incidence of alloantibody against the human platelet antigen (HPA) in pregnant women and its associations with NAIT in China.

METHODS

A multicentre, prospective cohort study design will be used, and 55,497 pregnant women will be recruited for the first screening of the anti-HPA antibody at 12 to 28 weeks of gestational age. Subjects who are positive in the first screening for the anti-HPA antibody will be included in the exposure group. Re-tests of the antibody titre, antigen-specificity and genotyping of HPA and HLA will be conducted during admission. A ratio of 1:1 paired individuals with the same ethnicity and parity but testing negative for the anti-HPA antibody will be randomly selected to be included in the non-exposure group. NAIT will be diagnosed in the newborns on day one of the birth. The HPA of the neonates in the exposure group will also be genotyped by sequencing. Associations of maternal HLA with the occurrence of the anti-HPA antibody and correlation of the severity of NAIT with the titre of the anti-HPA antibody will be further analysed.

DISCUSSION

The study is expected to provide baseline data on NAIT in China. Besides, we hope to find out a population who expresses particular HLA molecules has significant higher risk of HPA alloimmunization in Chinese individuals. We also hope to find a Chinese-specific cut-off antibody titre for the prediction of the severity of NAIT and to provide a means to evaluate the necessity of antenatal treatment.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT02934906 (date registered: 13.10.2016).

Low-dose prednisone and immunoglobulin G treatment for woman at risk for neonatal alloimmune thrombocytopenia and T helper 1 immunity

PROBLEM

Fetal and neonatal alloimmune thrombocytopenia is an alloimmune disorder resulting from platelet opsonization by maternal antibodies that destroy fetal platelets. As there is no antenatal screening or immunization to prevent sensitization, selection of high-risk population or the prevention of antenatal sensitization is significantly limited.

METHOD OF STUDY

(i) A case report of ante- and postnatal management of a woman with paternal homozygosity for human platelet antigen-1(HPA) incompatibility. (ii) A retrospective case-control study of 11 confirmed FNAIT patients, 8 possible-FNAIT women, and 10 women with confirmed ITP.

RESULT

Antenatal screening, prevention of maternal sensitization by serial monitoring and immunosuppression with prednisone and intravenous immunoglobulin G (IVIG) infusion resulted in two successful pregnancies without sensitization.

CONCLUSION

Screening for couples at risk and prednisone and/or IVIG treatment is an option for women with paternal homozygosity for offending HPA antigen to prevent antenatal sensitization. HPA incompatibility is associated with increased Th1 immunity and NK cell cytotoxicity.

T cell responses to human platelet antigen-1a involve a unique form of indirect allorecognition

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is a pregnancy-related condition caused by maternal antibodies binding an alloantigen on fetal platelets. In most cases the alloantigen is formed by a single amino acid, integrin β3 Leu33, referred to as human platelet antigen-1a (HPA-1a). Production of anti-HPA-1a antibodies likely depends on CD4⁺ T cells that recognize the same alloantigen in complex with the HLA-DRA/DRB3*01:01 molecule. While this complex is well characterized, T cell recognition of it is not. Here, to examine the nature of antigen recognition by HPA-1a-specific T cells, we assayed native and synthetic variants of the integrin β3 peptide antigen for binding to DRA/DRB3*01:01-positive antigen-presenting cells and for T cell activation. We found that HPA-1a-specific T cells recognize non-allogeneic integrin β3 residues anchored to DRA/DRB3*01:01 by the allogeneic Leu33, which itself is not directly recognized by these T cells. Furthermore, these T cell responses are diverse, with different T cells depending on different residues for recognition. This represents a unique form of indirect allorecognition in which a non-allogeneic peptide sequence becomes immunogenic by stable anchoring to MHC by an allogeneic residue.

Human platelet antigen (HPA)-1a peptides do not reliably suppress anti-HPA-1a responses using a humanized severe combined immunodeficiency (SCID) mouse model

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) occurs most frequently when human platelet antigen (HPA)-1a-positive fetal platelets are destroyed by maternal HPA-1a immunoglobulin (Ig)G antibodies. Pregnancies at risk are treated by administration of high-dose intravenous Ig (IVIG) to women, but this is expensive and often not well tolerated. Peptide immunotherapy may be effective for ameliorating some allergic and autoimmune diseases. The HPA-1a/1b polymorphism is Leu/Pro33 on β3 integrin (CD61), and the anti-HPA-1a response is restricted to HPA-1b1b and HLA-DRB3*0101-positive pregnant women with an HPA-1a-positive fetus. We investigated whether or not HPA-1a antigen-specific peptides that formed the T cell epitope could reduce IgG anti-HPA-1a responses, using a mouse model we had developed previously. Peripheral blood mononuclear cells (PBMC) in blood donations from HPA-1a-immunized women were injected intraperitoneally (i.p.) into severe combined immunodeficient (SCID) mice with peptides and HPA-1a-positive platelets. Human anti-HPA-1a in murine plasma was quantitated at intervals up to 15 weeks. HPA-1a-specific T cells in PBMC were identified by proliferation assays. Using PBMC of three donors who had little T cell reactivity to HPA-1a peptides in vitro, stimulation of anti-HPA-1a responses by these peptides occurred in vivo. However, with a second donation from one of these women which, uniquely, had high HPA-1a-specific T cell proliferation in vitro, marked suppression of the anti-HPA-1a response by HPA-1a peptides occurred in vivo. HPA-1a peptide immunotherapy in this model depended upon reactivation of HPA-1a T cell responses in the donor. For FNAIT, we suggest that administration of antigen-specific peptides to pregnant women might cause either enhancement or reduction of pathogenic antibodies.

Alloimmunization to transfused platelets requires priming of CD4+ T cells in the splenic microenvironment in a murine model

BACKGROUND

Alloantibodies are a clinically significant sequelae of platelet (PLT) transfusion, potentially rendering patients refractory to ongoing PLT transfusion support. These antibodies are often IgG class switched, suggesting the involvement of CD4+ T-cell help; however, PLT-specific CD4+ T cells have not been visualized in vivo, and specifics of their stimulation are not completely understood.

STUDY DESIGN AND METHODS

A murine model of alloimmunization to transfused PLTs was developed to allow in vivo assessment and characterization of CD4+ T cells specific for PLT major histocompatibility complex (MHC) alloantigen. PLTs were harvested from BALB/c mice, filter leukoreduced, and transfused into C57BL/6 recipients. PLT-specific CD4+ T-cell responses were visualized by using a T-cell receptor transgenic mouse that detects peptide from donor MHC I presented on recipient MHC II. Antibody responses were determined by indirect immunofluorescence using BALB/c donor targets.

RESULTS

C57BL/6 recipients of BALB/c leukoreduced PLT transfusions produced BALB/c antibodies, with proliferation of antigen-specific CD4+ T cells seen in the spleen but not lymph nodes or liver. Depletion of recipient CD4+ cells or splenectomy independently abrogated the alloantibody response.

CONCLUSION

We report a novel model to study antigen-specific CD4+ T cells during alloimmunization to PLT transfusion. The presented data support a critical role for CD4+ T-cell help in the humoral response to PLT transfusion and establish the spleen as a required microenvironment for effective CD4+ T-cell priming against donor PLT-derived MHC I.

Placental immunology and maternal alloimmune responses

During pregnancy, women are tolerant of their semi-allogeneic fetus whilst not being immunosuppressed and indeed readily form alloantibodies. This 'Immunological Paradox of Pregnancy' may be explained by an understanding of placental anatomy and immunology. Trophoblast cells form the interface between the fetus and maternal tissues and blood and escape allorecognition because they lack classical human leucocyte antigen (HLA) class I and II molecules. Local immunoregulation, or tolerance, in the decidua is mediated partly by HLA-G(+) extravillous trophoblasts (EVT) that invade the tissue and prevent killing by maternal natural killer cells, cytotoxic T cells and macrophages. Placental hormones orchestrate the composition and regulatory function of maternal immune cells. In contrast, syncytiotrophoblast cells at the surface of chorionic villi, in contact with maternal blood, maintain a state of mild maternal systemic immunity via activation of innate immunity and skewing towards humoral immunity. This enables maintenance of a healthy immune system in pregnant women and robust protective antibody responses to pathogens whilst enabling survival of the fetus. However, this has the unfortunate consequence that pregnant women readily form alloantibodies to incompatible alloantigens on fetal red cells, platelets and leucocytes if fetomaternal haemorrhage (FMH) occurs. The antibodies are initially low affinity but after re-immunization with further FMH become functionally effective, high-titre IgG.

The cellular immunobiology associated with fetal and neonatal alloimmune thrombocytopenia

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) is caused by maternal antibodies that cross the placenta in connection with pregnancy and destroy fetal platelets. Recently, maternal T cell responses associated with FNAIT have been studied at the clonal level. These T cell clones recognize an integrin β3 epitope, which is anchored to the HLA-DRB3∗0101-encoded MHC molecule DR52a. The same MHC allele is strongly associated with FNAIT. As the production of pathological antibodies reactive with fetal platelets is likely dependent on these T cell responses, there exists a potential for preventing FNAIT by targeting these T cells.

Reconsidering fetal and neonatal alloimmune thrombocytopenia with a focus on screening and prevention

Uncertainty regarding the pathophysiology of fetal and neonatal alloimmune thrombocytopenia (FNAIT) has hampered the decision regarding how to identify, follow-up and treat the women and children with this potentially serious condition. Since knowledge of the condition is derived mainly from retrospective studies, understanding of the natural history of this condition remains incomplete. General screening programs for FNAIT have still not been introduced, mainly because of a lack of reliable risk factors and effective treatment. Now, several prospective screening studies involving up to 100,000 pregnant women have been published and the results have changed the understanding of the pathophysiology of FNAIT and, thereby, the approach toward diagnostics, prevention and treatment in a more appropriate way.

Reassessing the role of HLA-DRB3 T-cell responses: evidence for significant expression and complementary antigen presentation

In humans, several HLA-DRB loci (DRB1/3/4/5) encode diverse beta-chains that pair with alpha-chains to form DR molecules on the surface of APC. While DRB1 and DRB5 have been extensively studied, the role of DRB3/4 products of DR52/DR53 haplotypes has been largely neglected. To clarify the relative expression of DRB3, we quantified DRB3 mRNA levels in comparison with DRB1 mRNA from the same haplotype in both B cells and monocytes, observing quantitatively significant DRB3 synthesis. In CD19+ cells, DRB1*03/11/13 was 3.5-fold more abundant than DRB3, but in CD14+ this difference was only two-fold. Monocytes also had lower overall levels of DR mRNA compared with B cells, which was confirmed by cell surface staining of DRB1 and DRB3. To evaluate the functional role of DRB3, tetramer-guided epitope mapping was used to detect T cells against tetanus toxin and several influenza antigens presented by DRB3*0101/0202 or DRB1*03/11/13. None of the epitopes discovered were shared among any of the DR molecules. Quantitative assessment of DRB3-tetanus toxin specific T cells revealed that they are present at similar frequencies as those observed for DRB1. These results suggest that DRB3 plays a significant role in antigen presentation with different epitopic preferences to DRB1. Therefore, DRB3, like DRB5, serves to extend and complement the peptide repertoire of DRB1 in antigen presentation.

Is it time to implement HPA-1 screening in pregnancy?

PURPOSE OF REVIEW

The purpose of the review is to argue for and against introduction of HPA-1 typing of all pregnant women to reduce morbidity and mortality caused by foetal/neonatal alloimmune thrombocytopenia (FNAIT).

RECENT FINDING

Several groups have done HPA-1 typing in cohorts of pregnant women. Results from a Norwegian study (>100,000 pregnancies) indicate that screening combined with simple intervention decreases morbidity and mortality due to FNAIT and is cost effective in Norway. Results from this study and several other studies show that there is correlation between the level of anti-HPA-1a antibodies in the mother and the severity of thrombocytopenia in the newborn. An important finding is that about 75% of women with antibodies are immunized in connection with delivery. Only 25% of the women are immunized during pregnancy.

SUMMARY

Screening for FNAIT does not fully meet the criteria presented by the WHO. Nevertheless, the results of the Norwegian study strongly indicate that morbidity and mortality related to FNAIT can be reduced. If the recent attempts to make a vaccine aimed at prevention of immunization and/or tolerizing peptides or neutralizing antibodies for already immunized women are shown to be successful, screening must be implemented.

Processed platelet HPA1a peptides au naturel

An understanding of how helper T cells are activated to drive blood group immune responses will help to explain the immunogenicity of antigens such as HPA-1a. In this issue of Blood, Anani Sarab and colleagues identify the naturally processed HPA1a peptides recognized by T cells and set the stage for peptide-based therapies in NAIT.(1).

Naturally processed peptides spanning the HPA-1a polymorphism are efficiently generated and displayed from platelet glycoprotein by HLA-DRB3*0101–positive antigen-presenting cells

In neonatal alloimmune thrombocytopenia, almost all human platelet antigen (HPA)–1b1b mothers who produce anti–HPA-1a antibody through carrying an HPA-1a fetus are human histocompatibility leukocyte antigen (HLA)–DRB3*0101 positive. It is predicted that the HPA-1a Leu33 polymorphism forms part of an HLA-DRB3*0101–restricted T-helper epitope, and acts as an anchor residue for binding this class II molecule. However, it is not known whether any corresponding peptides are naturally processed and presented from platelet glycoprotein. In this study, peptides displayed by a homozygous HLA-DRB3*0101 antigen-presenting cell line were identified after pulsing with recombinant HPA-1a (Leu33 plexin-semaphorin-integrin domain). The peptides were eluted from HLA-DR molecules, fractionated by high performance liquid chromatography, and analyzed by tandem mass spectrometry. A “nested set” of naturally presented HPA-1a–derived peptides, each containing the Trp25-Leu33 core epitope, was identified, with the most abundant member being the 16-mer Met22-Arg37. These peptides may provide the basis for novel treatments to tolerize the corresponding T-helper response in women at risk of neonatal alloimmune thrombocytopenia.

Evidence for the specificity for platelet HPA-1a alloepitope and the presenting HLA-DR52a of diverse antigen-specific helper T cell clones from alloimmunized mothers

Maternal alloantibodies against the human platelet Ag (HPA)-1a allotype of the platelet beta(3) integrin GpIIb/IIIa can cause severe fetal or neonatal hemorrhage. Almost all anti-HPA-1a-immune mothers are homozygous for HPA-1b and carry HLA-DR52a (DRB3*0101). The single Pro(33) -->Leu substitution (HPA-1b-->HPA-1a) was previously predicted to create a binding motif for HLA-DR52a that can lead to alloimmunization. We have isolated six CD4(+) T cell clones from three such mothers, which all respond to intact HPA-1a(+), but not HPA-1b(+), platelets. We used them to define the "core" and "anchor" residues of this natural T cell epitope. Molecular modeling based on a recently published crystal structure can explain the preferential presentation of the Leu(33) (but not Pro(33) variant) by HLA-DR52a rather than the linked HLA-DR3 or the allelic DR52b. The modeling also predicts efficient anchoring at position 33 by several alternative hydrophobic alpha-amino acids; indeed, a recently identified variant with Val(33) is presented well to two clones, and is therefore potentially alloimmunogenic. Finally, these HPA-1a-specific T cell clones use a variety of T cell receptors, but all have a "Th1" (IFN-gamma-producing) profile and are suitable for testing selective immunotherapies that might be applicable in vivo.

T-cell responses associated with neonatal alloimmune thrombocytopenia: isolation of HPA-1a-specific, HLA-DRB3*0101-restricted CD4+ T cells

T-cell responses have been implicated in the development of HPA-1a-induced neonatal alloimmune thrombocytopenia (NAIT). However, HPA-1a-specific T cells have neither been isolated nor characterized. Here, we aimed to determine whether HPA-1a-specific T cells could be isolated from HPA-1a-immunized women. In the present study, peripheral blood mononuclear cells (PBMCs) from an HPA-1a-alloimmunized woman were cultured for weeks in the presence of HPA-1a peptide, labeled with CFSE, and assayed for antigen-specific proliferation. Individual proliferating cells were isolated by fluorescence-activated cell sorting and expanded in culture. Antigen specificity and HLA restriction were determined by cytokine secretion (enzyme-linked immunospot [ELISPOT]) and proliferation assays. Several CD3(+)CD4(+) T-cell clones were isolated that proliferated and secreted cytokines in response to HPA-1a peptide. Two of these clones have been established in long-term culture in our laboratory. Both of these recognize synthetic as well as naturally processed HPA-1a antigen, and the recognition is restricted by the MHC molecule HLA-DRB3*0101 that is strongly associated with NAIT. These HPA-1a-specific T-cell clones represent unambiguous evidence for the association of T-cell responses with NAIT, and they will serve as unique tools to elucidate the cellular immune response that may result in NAIT.

The alloimmune response to the human platelet antigen-1a is not related to maternal-fetal killer immunoglobulinlike receptor/HLA-Cw combinations

BACKGROUND

Human platelet antigen (HPA)-1a fetomaternal alloimmune thrombocytopenia, responsible in the most severe cases for fetal or neonatal intracranial hemorrhages leading to death or survival with neurologic sequelae, was shown to be restricted to the human leukocyte antigen (HLA) Class II DRB3*0101-encoded molecule. Whereas more than 90 percent of alloimmunized mothers display the DRB3*0101 allele, the positive predictive value of the presence of DRB3*0101 is only 35 percent. Additional genetic risk factors may exist of which elucidation could improve the undertaking of incompatible pregnancies in at-risk families, encouraging an antenatal screening. Interactions of killer immunoglobulinlike receptors (KIRs) on maternal decidual NK cells with HLA-Cw molecules on fetal trophoblasts were reported as one of the mechanisms involved in the fetomaternal tolerance during pregnancy.

STUDY DESIGN AND METHODS

Genotyping was performed of 16 KIR genes in HPA-1a-negative/DRB3*0101-positive alloimmunized mothers and in HPA-1a-negative/DRB3*0101-positive nonimmunized mothers as well as HLA-Cw genotyping in thrombocytopenic children and their nonaffected siblings.

RESULTS

No particular KIR genes or KIR genotypes were observed in the alloimmunized or nonimmunized mothers. Distribution of HLA-Cw genes in affected infants and nonaffected siblings did not reveal any HLA-Cw specificity associated with triggering or modulation of the HPA-1a alloimmunization. No maternal KIR/fetal HLA-Cw combinations were demonstrated in association with a detrimental or a protective effect on the HPA-1a alloimmunization.

CONCLUSION

Maternal KIR/fetal HLA-Cw gene combinations that are involved in the fetomaternal tolerance do not appear to play a role in the HPA-1a alloimmunization.

Mapping helper T-cell epitopes on platelet membrane glycoprotein IIIa in chronic autoimmune thrombocytopenic purpura

Chronic autoimmune thrombocytopenic purpura (AITP) is associated with autoantibodies specific for platelet membrane components, often including glycoprotein GPIIIa. T helper (Th) cells reactive with GPIIIa, which are capable of driving the autoantibody response, are activated in AITP, and the aim here was to map the epitopes that they recognize. Peripheral blood mononuclear cells (PBMCs) were obtained from 31 patients with AITP and 30 control donors and stimulated with a panel of 86 overlapping synthetic 15-mer peptides spanning the complete sequence of GPIIIa. One or more peptides elicited recall proliferation by PBMCs from 28 of the patients, and, typically, multiple sequences were stimulatory. In contrast, responses in healthy control donors were rare (chi-square test = 115.967; P ≤ .001). It was confirmed that the proliferating PBMCs from patients were cells of the CD3+CD4+ helper phenotype that were MHC class II restricted. Despite variation between different cases of AITP, particular sequences were commonly recognized with PBMCs from 24 patients (77%) responding to 1 or more of the 4 most dominant peptides. Mapping such dominant autoreactive helper epitopes is the first step in the development of new approaches to the treatment of AITP, based on the use of peptides to tolerize Th cells specific for platelet glycoproteins.